Impact of Aqueous Electrolytes on Interfacial Energy

JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.

200, 172–181 (1998)

CS975380

Impact of Aqueous Electrolytes on Interfacial Energy Michael A. Butkus 1 and Domenico Grasso 2 Environmental Engineering Program, The University of Connecticut, Storrs, Connecticut 06269-2037 Received September 15, 1997; accepted December 8, 1997

The effects of aqueous electrolytes on particle–particle interactions in aqueous media are not completely understood. Electrolytes are typically considered to impact only electrostatic energy. In this work, the impact of aqueous electrolytes on van der Waals and Lewis acid/base potential energies was quantified using both hydrophobic and hydrophilic substrates. Increases in ionic strength beyond 0.001 M resulted in comparable increases in solid–water interfacial energy in polytetrafluoroethylene (PTFE) and organically coated sodium montmorillonite systems. The change in water/PTFE interfacial energy, as determined by contact angle measurements, was greater than that predicted by screening of the nondispersion component of the Hamaker constant using the Mahanty and Ninham model. Increases in interfacial energy, as a function of salt concentration, were greater with increasing substrate hydrophilicity. Although the systems studied in this work were disparate in nature, they all appeared to exhibit similar trends in interfacial surface energy as a function of ionic strength. Similarity in the results may suggest that aqueous electrolytes may modify the solvent structure within the interfacial region. q 1998 Academic Press

Key Words: contact angle; DLVO; electrolyte; Hamaker constant; Lewis acid/base interactions; surface tension; van der Waals interactions.

INTRODUCTION

The Derjaguin–Landau–Verwey–Overbeek model sensu stricto (DLVOSS ) has been used to describe particle–particle interactions for a good part of the 20th century (1, 2). DLVOSS modeling is predicated on a force (or energy) balance approach that can be used to predict particle stability in aqueous media. The model incorporates van der Waals (vdW) and electrostatic forces. For two identical particles in aqueous media, the vdW force is always attractive and dominates at close range and very large distances; the electrostatic force is typically repulsive and dominates at inter1

Present address: Department of Geography and Environmental Engineering, United States Military Academy, West Point, NY 10996-1695. 2 To whom correspondence should be addressed. E-mail: grasso@eng2. uconn.edu.

JCIS 5380

/

6g3f$$$421

BACKGROUND

While it is generally agreed that the air–water interface is hydrophobic (15), the electrostatic potential at this interface remains uncertain, as both positive and negative values have been reported (22–27). The structure of water at interfaces

172

0021-9797/98 $25.00 Copyright q 1998 by Academic Press All rights of reproduction in any form reserved.

AID

mediate distances. Unfortunately, the DLVOSS modeling approach has often proved inadequate in explaining particle– particle interactions in polar media (3–5). It has been suspected for many years (6–11) that components of surface tension, such as those related to solvent hydrogen bonding capabilities must also be considered. Van Oss and co-workers (12) have developed an extended DLVO model (DLVOE X ) which accommodates polar components of surface tension for both liquids and solids. These Lewis acid/ base (AB) interactions may be of quantitative significance in systems with electron donor/acceptor potentials. Investigations into the effects of salt on the interfacial properties of water have been conducted for over a century (13). Within the framework of the DLVOSS colloid stability model, an increase in electrolyte concentration is often correlated with a concomitant reduction in the extent of the electrostatic force. This typically results in increased attraction between two similar surfaces. However, cases have been reported in the literature which do not conform to this framework. For example, Yotsumoto and Yoon (14) reported a redispersion of rutile colloids, above a specific salt concentration, which resulted in an increase in suspension turbidity. Similarly, Craig et al. (15) observed that the stability of air bubbles increased with increases in aqueous electrolyte concentration. Moreover, the presence of an indifferent electrolyte has been reported to increase the energy available for coagulation beyond that predicted by the DLVOSS model (16–20). This work examines the effects of monovalent aqueous electrolytes on the vdW and AB components of solid–water interfacial tension. The impact of substrate hydrophilicity on ionic-strength-induced change in solid–water interfacial energy is discussed. Both experimental data, obtained as part of this study, and literature-reported data were utilized.

03-12-98 06:03:48

coidas

IMPACT OF AQUEOUS ELECTROLYTES ON INTERFACIAL ENERGY

is likely to be different from that in the bulk due to its restricted ability to hydrogen bond. Du et al. (28) reported that at the air–water interface, the surface water layer assumes a roughly hexagonal structure with the surface terminated by dangling OH bonds. These dangling OH bonds may in part account for the potentials reported for the air– water interface. Du et al. (28) also found that at the water– hydrophobic solid interface, the rigid wall results in a more ordered structure due to restrictions associated with packed molecules. The spatial extent of structured water near the interface is also a highly debated topic. Values range from a few to thousands of angstroms (29–31). The ordering of water is typically assumed to be affected by interface polarity and to decay exponentially away from the interface (23, 32–34). It has been presumed that this ordering results in ‘‘hydration’’ or ‘‘structural’’ forces which have been measured with atomic force apparatus (35, 36). It has also been proposed that aqueous electrolytes may disrupt this interfacial structure (37, 38) and thereby affect non-DLVO forces (i.e., forces other than electrostatic and vdW) (39, 40). The surface forces apparatus (41) and atomic force microscope (42) have allowed insight into interfacial phenomena, providing significant evidence that aqueous electrolytes play a role in non-DLVO interactions (27, 43–48). Using a surface forces apparatus, Pashley (43, 44) measured the force between two mica surfaces in electrolyte solutions and reported that deviations from DLVO forces were observed and noted to be a function of electrolyte type and concentration. These observations were related to the ion exchange properties of the mica surface and the waters of hydration surrounding the ion. The salt was reported to sorb onto the mica surface, which resulted in a repulsive hydration force above a salt-specific concentration. Using neutron diffraction, Leberman and Soper (49) found that salt effects on water of hydration were also ion specific. Pashley also noted that hydration forces appeared to approach an asymptotic value above a salt-specific concentration. The etiology of this observation was attributed to saturation of the charged mica surface sites. Experimental findings below the saltspecific concentration were reported to be consistent with the DLVOSS model. Pashley and Quirk (38) reported that the net hydration force, measured between mica surfaces in high concentrations of NaCl, was significantly lower than the hydration force measured in NaCl solutions which contained only enough electrolyte to fully cover the mica surfaces. More recently, Colic et al. (50) reported that the shortrange repulsion, observed between surfaces in electrolyte solutions, may be due to counterions located near the surface (i.e., not necessarily adsorbed) and is a function of the bare ion radius. The effects of strong acids and strong bases on the surface tension of water have also been investigated. Addition of

AID

JCIS 5380

/

6g3f$$$422

03-12-98 06:03:48

173

inorganic acids to water appears to have a small effect on the total surface tension of water. Work by Ha˚rd and Johansson (51) suggested that the conjugate base of strong acids and not the proton may be responsible for decreases in surface tension. Addition of strong bases has been reported to have an effect that is similar to that of electrolytes (15, 51). Work by Weissenborn and Pugh (52) suggested that the conjugate acid of a strong base is responsible for increases in the surface tension of water. In addition, data presented by Ha˚rd and Johansson (51) suggest that the increase in water surface tension caused by the conjugate acid of a strong base is much more pronounced than the decrease in surface tension caused by the conjugate base of a strong acid. Furthermore, it appears that the cation is responsible for the electrolyte effect on DLVOEX interactions between negatively charged surfaces (43). The pH of a solution appears to have two distinct indirect effects on the interfacial energy between surfaces. First, Pashley (44) reported that pH indirectly affected hydration force by changing the surface charge, thereby affecting the magnitude of salt adsorption. Pashley (44) also noted that sorption of the H3O / ion did not result in a repulsive hydration force between bubbles in acidic solutions, which is similar to the findings of Craig et al. (15). Changes in solution pH may affect the solid–water interfacial energy by changing the characteristics of surface functional groups. For example, Holmes-Farley et al. (53) reported that lowering the pH of water drops, which were placed on a polyethylene carboxylic acid (PE–CO2H) surface, increased the hydrophobicity of the surface (due to protonation of the surface groups) and resulted in increased water contact angles. Similarly, van Oss (34) reported that Lewis acid/base sites (which affect AB forces) are affected by pH due to protonation of these sites. THEORY

Surface energies of solid surfaces can be calculated via contact angle measurements with liquids of known surface tension properties (54), gsw 0 gsv / gwv cos u Å 0,

[1]

where gsw is the solid–water surface tension in J/m 2 , gsv is the solid–vapor surface tension in J/m 2 , gwv is the water– vapor surface tension in J/m 2 , and u is the contact angle in degrees. Substitution of the Dupre´ equation (55) into Eq. [1] yields the relationship 0DGsvw Å gwv (1 / cos u ),

coidas

[2]

174

BUTKUS AND GRASSO

where DGsvw is the free energy of adhesion between the solid and water in J/m 2 . Known as the Young–Dupre´ equation, this relationship allows one to determine the solid–water interfacial free energy as a function of contact angle and the surface tension at the water–vapor interface. It has been proposed that the free energy of adhesion can be separated into components which are additive (8). These components include, but are not limited to, dispersion, dipolar, induction, and H-bonding (34). Using a geometric combining rule, Girifalco and Good (56) and Fowkes (8) developed the following relationship for the vdW component of interfacial tension: q

vdW g vdW / g vdW 0 2 g vdW g vdW sw Å g s w s w .

[3]

The subscripts on the terms on the left-hand side of Eq. [3] are singular, which, in the true sense, implies that these terms correspond to the indicated phase in vacuo. Bangham and Razouk (57) defined the following relationship for equilibrium film pressure ( p ), when considering a wetting fluid on a solid: p Å gs 0 gsv .

[4]

The last term on the right-hand side of Eq. [4] accounts for the free energy when the solid is in equilibrium with the saturated vapor of the wetting liquid. Many workers (58– 60) have reported that the effects of spreading pressure can be neglected when gw ú gs . In addition, the subscript (v), which indicates that the energy corresponds to a gaseous medium, is often omitted and will be here after. The free energy associated with dispersion forces between two identical materials in water can be calculated using the Lifshitz continuum theory (61, 62), DG LW 1w1 Å

0 A1w1 , 12ph 20

[5]

where A1w1 is the nonretarded Hamaker constant between two identical phases in water, w, J; and h0 is the distance of closest approach (34). The nonretarded Hamaker constant is composed of nondispersion (AnÅ0 ) and dispersion (Anú0 ) components, such that (63) A Å AnÅ0 / Anú0 .

AID

JCIS 5380

/

6g3f$$$423

03-12-98 06:03:48

D

3 e1 0 e3 kT 4 e1 / e3

Anú0 Å

3hqne (n 21 0 n 23 ) 2 , 2 2 3/2 16 2 (n 1 / n 3 )

[7] [8]

where k is Boltzmann’s constant Å 1.381 1 10 023 J K 01 , T is the absolute temperature in Kelvin, e is the dielectric constant of material, h is Planck’s constant Å 6.626 1 10 034 J s, ne is the dominant electronic absorption frequency in the UV range (62), and n is the refractive index of material. Israelachvili (62) reported that, in many cases, the approximations given by Eqs. [7] and [8] yield results very similar to those of more rigorous relationships developed by Hough and White (64). Van Oss et al. (12) defined the polar component of free energy of cohesion between two identical materials in a vacuum and adhesion between water and a solid in air, respectively, as q

/ 0 DG AB 11 Å 04 g 1 g 1

q

[9] q

/ 0 g s0 g w/ ), DG AB sw Å 02( g s g w /

[10]

where the / / 0 surface tension terms imply the electron acceptor and electron donor components of surface tension, respectively. Assuming the surface tension components are additive, van Oss et al. (12) proposed (1 / cos u ) gwv q

q

q

LW Å 2( g LW g s/ g w0 / g s0 g w/ ). s gw /

[11]

Van Oss et al. (65) have defined the g / and g 0 terms for water equal to 25.5 1 10 03 J/m 2 . AB parameters for other solids and liquids can be determined based on the operationally defined AB values for water in conjunction with Eqs. [5] – [8] and [11]. The free energy of interaction between two identical particles, 1, in water, w, can be determined using the DLVOEX model (12): q

q

2 DG1w1 Å 02( g LW 0 g LW 1 w )

q

q

q

q

0 4( g 1/ g 10 / g w/ g w0 0 g 1/ g w0 0 g 10 g w/ ).

[12]

The free energy of interaction is related to the interfacial free energy via the Dupre´ equation:

[6]

The nonretarded Hamaker constant, for two identical materials in air, can be approximated from material properties, viz. (62)

S

2

AnÅ0 Å

DG1w1 Å 02gsw .

[13]

The presence of an electrolyte has been found to decrease the magnitude of the dispersion force (63). Electrolytes may screen electrostatic fields in the medium, which can result

coidas

IMPACT OF AQUEOUS ELECTROLYTES ON INTERFACIAL ENERGY

175

FIG. 1. Effect of ionic strength (NaCl) on water contact angles measured on PTFE; pH 4.85, error bars are 95% confidence interval. The inset provides a logarithmically expanded view of the abscissa.

in a reduction in the Hamaker constant and, therefore, a reduction in the vdW component of surface tension (cf. Eqs. [5] – [8] for the relationship between the Hamaker constant and the vdW surface tension component). Mahanty and Ninham (63) have reported that salt affects only the nondispersion portion of the Hamaker constant. The dispersion component remains unscreened because the electrolyte cannot respond to high frequencies ((63); see also Ref. (62)). For the limit where kh0 ! 1, the effect of the electrolyte on the Hamaker constant between two water drops in vacuo (or air) is given by (63)

H

A Å Anú0 / AnÅ0 1 0 (2kh0 ) 2[ 0ln(2kh0 ) / 0.5722]

2e 21 / O[( kh0 ) 3 ln( kh0 )] e 0 e 22 2 1

J

.

[14]

k is the reciprocal of the Debye length and for a monovalent electrolyte can be written as (see Ref. (66)) k Å 3.281 1 10 9 (I) 1 / 2 ,

m01 ,

[15]

where I is the ionic strength.

AID

JCIS 5380

/

6g3f$$$423

03-12-98 06:03:48

Equation [14] is based on the assumption that surface charge effects on aqueous electrolyte distribution (diffuse double layer effects) can be neglected when considering screening of the van der Waals interaction. This assumption will also be made here. MATERIALS AND METHODS

Contact angle measurements were conducted using a Rame´ –Hart goniometer on both hydrophobic and hydrophilic surfaces. Contact angle measurements were conducted following the method outlined by van Oss (34). Advancing contact angles were measured, as they are assumed to represent thermodynamic equilibrium and are therefore extensible to the Young equation (34, 67). A minimum of 20 contact angle measurements were averaged for each reported measurement. All measurements were conducted at 257C. PTFE (produced by Interplast, New Jersey) was machined on a lathe to form a smooth flat surface which minimized the effects of surface roughness. Resurfacing should decrease the effects reported by Miller et al. (68); that is, increased surface roughness of PTFE resulted in increased contact angles. However, machining a PTFE surface can result in an increase in heterogeneity (59). Consequently, in

coidas

176

BUTKUS AND GRASSO

FIG. 2. Effect of ionic strength (NaCl) on the normalized Hamaker constant for water ( A0 is the Hamaker constant of DI water). The experimental points are based on water contact angles measured on PTFE (Fig. 1) as a function of salt concentration. The line depicts screening of the nondispersion portion of the Hamaker constant as predicted by Eq. [14]. The inset provides a logarithmically expanded view of the abscissa.

order to obtain statistically representative values numerous contact angle measurements were required. It should be pointed out that the main objective of this work is to determine the change in contact angle as a function of salt concentration. Thus, these interferences are expected to self-compensate. The contact angle measurements were carried out using salt solutions of various ionic strengths. Salt solutions were prepared in 100-ml volumetric flasks using certified A.C.S. sodium chloride supplied by Fisher Scientific. PTFE surfaces were soaked in DI water, air dried, and washed in pentane (or hexane), followed by a final wash in acetone. The samples were then stored overnight in a desiccator. All solvents were supplied by Fisher Scientific. Solution pH was adjusted using diluted reagent grade hydrochloric acid supplied by J. T. Baker or diluted sodium hydroxide supplied by Fisher Scientific. All pH measurements were obtained with a Fisher Scientific Accumet 925 pH meter using a combination glass electrode (Fisher 13620-285). Solutions were prepared with DI (Milli-Q) water. Labware was washed, soaked overnight in 10% HNO3 , and soaked in DI water.

AID

JCIS 5380

/

6g3f$$$424

03-12-98 06:03:48

RESULTS AND DISCUSSION

Experiments were conducted to examine the effects of ionic strength on the vdW and AB components of water in the interfacial region. These findings may provide insight into particle–particle interactions at high ionic strengths where the DLVOSS model typically diverges from experimental observations. Both experimental data, obtained as part of this study, and literature-reported data were utilized. In the first experiment, we measured contact angles of salt solutions on a hydrophobic surface (PTFE). Use of a hydrophobic surface (no AB component of surface tension) provided insight into the impact of aqueous electrolyte on the vdW component of water surface tension. The second experiment entailed measurement of contact angles formed by salt solutions on a hydrophilic surface (16). The findings reported here illustrate the simultaneous effect of ionic strength on vdW and AB components of water surface tension. By combining the results from the first two experiments, we estimated the impact of ionic strength on the AB component of water surface tension.

coidas

IMPACT OF AQUEOUS ELECTROLYTES ON INTERFACIAL ENERGY

177

FIG. 3. Effect of ionic strength on solid–water interfacial energy, as predicted by the Young equation, on cNa-M (16) and PTFE. An increase in the ozone dose represents an increase in the hydrophobicity of the cNa-M. Experimental points were fit with an equation of the form Dgsw Å A log(I) / B.

Figure 1 illustrates the effects of NaCl aqueous concentration, reported as ionic strength, on contact angles measured on PTFE. Similarly, Derjaguin and Churaev (39) reported an increase in contact angles of KCl solutions on quartz. The slope of the curve appears to change significantly near an ionic strength of approximately 0.02 M. This concentration is similar to Pashley’s critical hydration concentration for Na / (0.02 M) on Mica surfaces ((44); see also Ref. (69)). Given that the surface properties of the PTFE do not change with increases in ionic strength, the observed increase in contact angle results in an increase in gsw (cf. Eq. [1]). Because PTFE is hydrophobic, the observed change in gsw is presumed to be due to only vdW. We measured similar increases in contact angle, between DI water and 0.5 M NaCl, on paraffin wax (Parafilm, American National Can, Greenwhich, CT) and polyethylene, both of which are hydrophobic materials. The effects of ionic strength on the vdW interaction energy have been addressed by other researchers (63, 70–75). Reports of change in vdW, as a function of the electrolyte, have been attributed to screening of the nondispersion portion of the Hamaker constant (63, cf. Eq. [14]). Figure 2

AID

JCIS 5380

/

6g3f$$$424

03-12-98 06:03:48

compares the effect of NaCl on the Hamaker constant for water, as predicted by the Mahanty and Ninham model, with that calculated from contact angle measurements. Given that PTFE is not composed of polar functional groups, its vdW component was calculated to be 22.3 mJ/m 2 using Eqs. [2] and [3] and the contact angle data for DI water on PTFE (Fig. 1). The vdW component of surface tension for DI water was assumed to be 21.6 mJ/m 2 (34). The experimental points on Fig. 2 were calculated via Eqs. [2], [3], and [5] using change in contact angle as a function of NaCl concentration as the independent variable. The distance of closest approach was taken to be 0.157 nm (34). The model predictions of the Hamaker constant as a function of ionic strength were determined via Eqs. [6] – [8], [14], and [15]. Parameters for Eqs. [7] and [8] are given in Hough and White ((64); see also Ref. (62)). It should be pointed out that calculation of the Hamaker constant using Lifshitz theory resulted in a value slightly lower than that used for the experimental data. However, our objective is to report on the change in Hamaker constant as a function of ionic strength, rather than its absolute value. The total surface tension of water as a function of salt concentration was

coidas

178

BUTKUS AND GRASSO

FIG. 4. Change in solid–water interfacial energy as a function of ionic strength on cNa-M (16) and PTFE.

taken from (52). Figure 2 indicates that the decrease in the Hamaker constant as determined from contact angle measurements is much greater than that predicted by the Mahanty–Ninham model. Screening, by NaCl, of the nondispersion portion of the Hamaker constant does not appear to be the sole contributor to the total change in gsw , as depicted by the PTFE experimental data. These results suggest that the electrolyte may effect the interfacial energy in some other manner. Recently, Ninham and Yaminsky (4) reported that ions experience a potential near an interface, which is a function of their polarizability. This phenomenon is typically not accounted for in vdW calculations. Furthermore, Ninham and Yaminsky ((4); see also Ref. (5)) reported that the vdW and electrostatic energies are ‘‘inextricably tangled’’ and may not be separated into individual components. The inability of either DLVOSS or DLVOEX to address these interactions may compromise their extensibility in certain instances. We calculated gsw , as predicted by the Young equation as a function of NaCl concentration, on both hydrophobic

AID

JCIS 5380

/

6g3f$$$424

03-12-98 06:03:48

(PTFE) and hydrophilic (16) substrates (Fig. 3). The hydrophilic substrate was sodium montmorillonite (cNa-M) coated with natural autochthonous organic matter. Chheda and Grasso (16) studied the effect of ozone dose on the change in polarity of this colloidal material in environmental systems. They reported that the hydrophobicity of cNa-M increased with increased ozone dose. Figure 3 indicates that gsw increased, in all cases, as a function of ionic strength. Figure 4 illustrates the change in solid–water interfacial energy ( Dgsw ) as a function of ionic strength. Although the magnitude of gsw was quite different in PTFE and cNa-M, the Dgsw was similar on all surfaces. Figure 4 also illustrates that the effect of the salt was most significant on the most hydrophilic substrate. PTFE, which cannot hydrogen bond, was least affected by the presence of the salt. These findings are similar to those reported by Veeramasuneni et al. (76) for contact angle measurements made on francolite, a-alumina, and quartz using saturated solutions of NaCl and KCl. All curves appear to have the same general shape and appear to have a change in slope at (albeit at different ultimate

coidas

IMPACT OF AQUEOUS ELECTROLYTES ON INTERFACIAL ENERGY

FIG. 5. Effect of ionic strength on Lewis acid–base parameters for water measured on cNa-M. The increase in ozone dose represents an increase in the hydrophobicity of the cNa-M (16). (a) Change in the electron acceptor parameter for water. (b) Change in the electron donor parameter for water. Experimental points were fit with an equation of the form Dgsw Å A log(I) / B.

energies) ca. ionic strengths of 0.01 M. Similarity between the results for PTFE and cNa-M (surfaces which have significantly different characteristics) suggests that the electrolyte may be impacting the properties of the solvent rather than those of the surface. By combining the results from PTFE and cNa-M we estimated the effects of ionic strength on the AB components of water. Surface tension values (measured with diiodomethane, glycerol, and DI water) for cNa-M, as reported by Chheda and Grasso (16), were used in conjunction with Eq.

AID

JCIS 5380

/

6g3f$$$424

03-12-98 06:03:48

179

[11] to determine ionic strength effects on the electron donor/acceptor parameters for water. The total surface tension of water as a function of NaCl concentration was adjusted based on the correlation reported by Weissenborn and Pugh (52). The vdW component of water surface tension as a function of salt concentration was estimated from the contact angle measurements for PTFE presented in Fig. 2. The polar component of water surface tension as a function of salt concentration was determined by taking the difference between the total surface tension and the vdW component. Using contact angles measured on the cNa-M surfaces as a function of salt concentration (77), the AB parameters for water were determined by solving Eq. [11]. We assumed that the salt behaved as an ideal indifferent electrolyte. Thus, the cNa-M surface tension components were assumed constant as a function of ionic strength. The validity of this assumption will be discussed below. Figures 5a and 5b depict the change in electron acceptor and donor parameters for water in the interfacial region, respectively, as a function of ionic strength on a cNa-M surface. In this case, ionic strength appears to have a greater impact on the electron donor parameter for water. The figures also indicate that the hydrophilicity of the surface has an impact on how ionic strength affects the AB parameters; an increase in hydrophilicity resulted in a greater change in g / and g 0 . Du et al. (28) reported that the structure of water at an interface is a function of the surface hydrophilicity. The presence of salt in the interfacial region of water oriented near a hydrophilic surface may have a greater impact on the water’s ability to form hydrogen bonds and thereby result in a greater impact on the AB parameters than on water oriented near a hydrophobic surface. An alternative explanation for the observations depicted in Figs. 5a and 5b may be that the salt adsorbed onto the surface and blocked polar functional groups. It has been reported that indifferent electrolytes can adsorb onto surfaces (43, 44, 78), which should, as predicted by the Gibbs Isotherm equation, result in a decrease in interfacial energy. However, it is unlikely that the salt adsorbed onto the surfaces discussed here because the presence of the salt resulted in an increase in interfacial energy (R. M. Pashley, Australian National University, Canberra, Australia, personnel communication; see also Fig. 4). Furthermore, significant desorption of salt from the cNa-M surface was not expected, based on Le Chaˆtelier’s principle. Given that mass transfer of salt between the surface and the bulk was assumed negligible, it is plausible to assume that the vdW and AB surface tension parameters of the cNa-M remained constant as a function of ionic strength, i.e., indifferent electrolyte. These factors may indicate that the observed increase in gsw is at least partially a solvent-oriented effect. Furthermore, this effect may be manifested in surface tension measurements when electrolytes are present in the interfacial region.

coidas

180

BUTKUS AND GRASSO

A definitive conclusion has yet to be drawn regarding the mechanism of these observations. Similarity between the results for PTFE and cNa-M may indicate that the effects of aqueous electrolyte on interfacial phenomena are caused by changes in solvent properties near the interface rather than being strictly a function of surface properties, as reported by others (31). Salt may change the ordering of water dipoles at (or near) the surface, which may result in a decrease in interfacial entropy (49). A decrease in entropy which is not compensated for by a decrease in enthalpy will result in an increase in the free energy. Equation [9] indicates that an increase in the free energy of cohesion, due to an increase in salt concentration, should yield an increase in gsv , which is consistent with our findings and literature reports (79). SUMMARY

Aqueous electrolytes are ubiquitous in natural and engineered systems. To date, only a fraction of their effects on colloid stability have been considered. Salt affects both DLVOSS and DLVOEX energy components in aqueous media. In this work, the effect of indifferent electrolytes on solid–water interfacial energy was studied for both hydrophobic and hydrophilic substrates. The presence of electrolyte ions, near the solid–water interface, resulted in an increase in solid–water interfacial energy. The change in these properties as a function of ionic strength may be affected by the hydrophilicity of the substrate due to orientation between the water and the solid surface. The fact that similar behavior was observed in all cases may suggest that electrolyte ions impact the vdW and AB components of water and not those of the surface. ACKNOWLEDGMENTS The authors thank the following for their helpful comments during the preparation of this manuscript: Professors B. W. Ninham and R. M. Pashley, Australian National University, Canberra, Australia; Professor C. J. van Oss, State University of New York at Buffalo; and Professor J. F. Rusling, Department of Chemistry, University of Connecticut. The authors also thank Professor J. P. Bell and Mr. J. Soracchi, Institute of Material Science, University of Connecticut, for providing the PTFE and polyethylene samples. This research was funded, in part, by the South Central Connecticut Regional Water Authority, New Haven, CT.

REFERENCES 1. Derjaguin, B. V., and Landau, Acta Physicochim. U.S.S.R. 14, 633 (1941). 2. Verwey, E. J. W., and Overbeek, J. Th. G., ‘‘Theory of the Stability of Lyophobic Colloids.’’ Elsevier, Amsterdam, 1948. 3. Swanton, S. W., Adv. Colloid Interface Sci. 54, 129 (1995). 4. Ninham, B. W., and Yaminsky, V., Langmuir 13, 2097 (1997). 5. Ninham, B. W., Kurihara, K., and Vinogradova, O. I., Colloids Surf. A 123, 7 (1997). 6. Onsager, L., and Samaras, N. N. T., J. Chem. Phys. 2, 528 (1934).

AID

JCIS 5380

/

6g3f$$$425

03-12-98 06:03:48

7. Langmuir, I., J. Chem. Phys. 6, 873 (1938). 8. Fowkes, F. M., Ind. Eng. Chem. 56, 12, 41 (1964). 9. Derjaguin, B. V., and Churaev, N. V., J. Colloid Interface Sci. 49, 2, 249 (1974). 10. Kollman, P., J. Am. Chem. Soc. 99, 15, 4875 (1977). 11. Jensen, W. B., Chemtech 12, 755 (1982). 12. van Oss, C. J., Chaudhury, M. K., and Good, R. J., Chem. Rev. 88, 927 (1988). 13. Horvath, A. L., ‘‘Handbook of Aqueous Electrolyte Solutions; Physical Properties, Estimation and Correlation Methods.’’ Wiley, New York, 1985. 14. Yotsumoto, H., and Yoon, R. H., J. Colloid Interface Sci. 157, 426 (1993). 15. Craig, V. S. J., Ninham, B. W., and Pashley, R. M., J. Phys. Chem. 97, 10192 (1993). 16. Chheda, P., and Grasso, D., Langmuir 10, 1044 (1994). 17. Wu, W., Giese, R. F., and van Oss, C. J., Colloids Surf. A 89, 241 (1994). 18. Wu, W., Giese, R. F., and van Oss, C. J., Colloids and Surfaces A: Physicochemical and Engineering Aspects. 89, 253 (1994). 19. Grasso, D., Carrington, J. C., Chheda, P., and Kim, B., Wat. Res. 29, 49 (1995). 20. Butkus, M. A., ‘‘Impact of Phosphate on the Surficial Properties of a Ferric Hydroxide Matrix: Linkage Between Surface Complexation and Colloid Stability,’’ Ph.D. thesis. The University of Connecticut, Storrs, CT, 1997. 21. Borazio, A., Farrell, J. R., and McTigue, P., J. Electroanal. Chem. 193, 103 (1985). 22. Ushi, S., and Sasaki, H., J. Colloid Interface Sci. 65, 1, 36 (1978). 23. Churaev, N. V., and Derjaguin, B. V., J. Colloid Interface Sci. 103, 2, 542 (1985). 24. Duncan-Hewitt, W. C., Langmuir 7, 1229 (1991). 25. Li, C., and Somasundaran, P., J. Colloid Interface Sci. 146, 1, 215 (1991). 26. Churaev, N. V., Adv. Colloid Interface Sci. 58, 87 (1995). 27. Ducker, W. A., Zhenghe, X., and Israelachvili, J. N., Langmuir 10, 3279 (1994). 28. Du, Q., Freysz, E., and Shen, Y. R., Science 264, 826 (1994). 29. Drost-Hansen, W., Ind. Eng. Chem. 57, 4, 18 (1965). 30. Horn, R. A., in ‘‘Water and Water Pollution Handbook’’ (L. L. Ciaccio, Ed.), p. 915. Decker, New York, 1972. 31. Israelachvili, J., and Wennerstro¨m, H., Nature 379, 18, 219 (1996). 32. Belaya, M. L., Feigel’man, M. V., and Levadny, V. G., Langmuir 3, 648 (1987). 33. Marrink, S., Berkowitz, M., and Berendsen, J. C., Langmuir 9, 11, 3122 (1993). 34. van Oss, C. J., ‘‘Interfacial Forces in Aqueous Media.’’ Dekker, New York, 1994. 35. Gruen, D. W. R., and Marcelja, S., J. Chem. Soc. Faraday Trans 2, 79, 225 (1983). 36. Cevc, G., J. Chem. Soc. Faraday Trans. 87, 17, 2733 (1991). 37. Sposito, G., and Prost, R., Chem. Revs. 82, 6, 553 (1982). 38. Pashley, R. M., and Quirk, J. P., Colloids and Surfaces 9, 1 (1984). 39. Derjaguin, B. V., and Churaev, N. V., Langmuir 3, 5, 607 (1987). 40. Baygents, J. C., and Saville, D. A., J. Chem. Soc. Faraday Trans. 87, 1883 (1991). 41. Israelachvili, J. N., and Adams, G. E., J. Chem. Soc. Faraday Trans. I 74, 975 (1978). 42. Ducker, W. A., Senden, T. J., and Pashley, R. M., Nature 353, 239 (1991). 43. Pashley, R. M., J. Colloid Interface Sci. 80, 1, 153 (1981). 44. Pashley, R. M., J. Colloid Interface Sci. 83, 2, 531 (1981).

coidas

IMPACT OF AQUEOUS ELECTROLYTES ON INTERFACIAL ENERGY 45. Christenson, H. K., Claesson, P. M., and Parker, J. L., J. Phys. Chem. 96, 6725 (1992). 46. Meagher, L., J. Colloid Interface Sci. 152, 1, 293 (1992). 47. Kurihara, K., and Kunitake, T., J. Am. Chem. Soc. 114, 10927 (1992). 48. Toikka, G., Hayes, R. A., and Ralston, J., Langmuir 12, 3783 (1996). 49. Leberman, R., and Soper, A. K., Nature 378, 23, 364–366 (1995). 50. Colic, M., Franks, G. V., Fisher, M. L., and Lange, F. F., Langmuir 13, 3129 (1997). 51. Ha˚rd, S., and Johansson, K., J. Colloid Interface Sci. 60, 3, 467 (1977). 52. Weissenborn, P. K., and Pugh, R. J., J. Colloid Interface Sci. 184, 550 (1996). 53. Holmes-Farley, S. R., Reamey, R. H., McCarthy, T. J., Deutch, J., and Whitesides, G. M., Langmuir 1, 6, 725 (1985). 54. Young, T., ‘‘Miscelleneous Works,’’ (G. Peacock, Ed.), Vol. 1. Murray, London, 1855. 55. Dupre´, A., ‘‘The´orie Me´canique de la Chaleur,’’ p. 367. GauthierVillars, Paris, 1869. 56. Girifalco, L. A., and Good, R. J., J. Phys. Chem. 61, 904 (1957). 57. Bangham, D. H., and Razouk, R. I., Trans. Faraday Soc. 33, 1459 (1937). 58. Fowkes, F. M., McCarthy, D. C., and Mostafa, M. A., J. Colloid Interface Sci. 78, 200 (1980). 59. Good, R. J., J. Colloid Interface Sci. 59, 3, 398 (1977). 60. van Oss, C. J., Colloids Surf. A: Physicochem. Eng. Aspects 78, 1 (1993). 61. Hamaker, H. C., Physica 4, 1058 (1937). 62. Israelachvili, J. N., ‘‘Intermolecular and Surface Forces.’’ Academic Press, New York, 1992.

AID

JCIS 5380

/

6g3f$$$425

03-12-98 06:03:48

181

63. Mahanty, J., and Ninham, B. W., ‘‘Dispersion Forces.’’ Academic Press, New York, 1976. 64. Hough, D. B., and White, L. R., Advan. Colloid Interface Sci. 14, 3 (1980). 65. van Oss, C. J., Chaudhury, M. K., and Good, R. J., Advan. Colloid Interface Sci. 28, 35 (1987). 66. Stumm, W., ‘‘Chemistry of the Solid–Water Interface.’’ Wiley, New York, 1992. 67. Good, R. J., Surface Colloid Science 11, 1 (1979). 68. Miller, J. D., Veeramasuneni, S., Drelich, J., and Yalamanchili, M. R., Polymer Eng. and Sci. 36, 14, 1849 (1996). 69. Claesson, P. M., Herder, P., Stenius, P., Eriksson, J. C., and Pashley, R. M., J. Colloid Interface Sci. 109, 1, 31 (1986). 70. Davies, B., and Ninham, B. W., J. Chem. Phys. 56, 12, 5797 (1972). 71. Marra, J., J. Colloid Interface Sci. 109, 1, 11 (1985). 72. Mishchuk, N. A., Sjoblom, J., and Dukhin, S. S., Colloid J. 57, 6, 785 (1995). 73. Bowen, W. R., and Jenner, F., Adv. Colloid and Interface Sci. 56, 201 (1995). 74. Yaminsky, V. V., Ninham, B. W., Christenson, H. K., and Pashley, R. M., Langmuir 12, 1936 (1996). 75. Bowen, W. R., and Williams, P. M., J. Colloid Interface Sci. 184, 241 (1996). 76. Veeramasuneni, S., Hu, Y., Yalamanchili, M. R., and Miller, J. D., J. Colloid Interface Sci. 188, 473 (1997). 77. Chheda, P., ‘‘Impact of Ozone on Colloidal Stability: Surface Thermodynamics,’’ Ph.D. thesis. The University of Connecticut, Storrs, CT, 1994. 78. Parks, G. A., J. Geophys. Res. 89, B6, 3997 (1984). 79. Stairs, R. A., Can. J. Chem. 73, 781 (1995).

coidas